PXE over VPN, one way problem

I have two sites with a TFTP server per site and an openVPN tunnel managed by tomato.

In short the issue is: PXE locally within the local LAN for both Site1 and Site2 works fine, and even across the VPNs from Site1 to Site2. The problem is PXE from Site2 to Site1. The error I get is PXE-E35 please see image in attachment.

Where on the server side I can see a double session opened by the client.

I captured the relevant packets with wireshark, you can find them in attachment.
I initially thought about an MTU issue but it doesn't appear to be the problem...